FIG. 5 illustrates a switching power source device that improves an input power factor using a choke input-type smoothing circuit, as disclosed in Japanese Unexamined Patent Publication Hei-9 (1997)-131055. In the embodiment of FIG. 5, the switching power source device includes an AC power source 1, a noise filter 2, which is constituted of a reactor 3 and a capacitor 4, a bridge rectifying circuit 5, a smoothing capacitor 6, a transformer 7, a primary-side (hereafter “primary”) winding 7a of the transformer 7, a secondary-side (hereafter “secondary”) winding 7b of the transformer 7, a first switching element 8, a diode 9, a smoothing capacitor 10, a load 11, a control circuit 12 that performs an ON-OFF control of the switching element 8, resistances 13a, 13b for voltage detection, and a choke coil 17.
In the above-mentioned embodiment, the AC voltage supplied from the AC power source 1 is subjected to the full-wave rectification by the bridge rectifying circuit 5 through the noise filter 2. The full-wave rectified voltage output from the bridge rectifying circuit 5 is smoothed by the choke-input type smoothing circuit, which is constituted of the choke coil 17 and the smoothing capacitor 6. The first primary winding 7a of the transformer 7 and the switching element 8 are connected in series, and these elements are connected to both ends of the capacitor 6. The voltage smoothed by the smoothing circuit is interrupted by turning ON and OFF the switching element 8. The interrupted voltage is smoothed by the diode 9 and the smoothing capacitor 10 through the secondary winding 7b of the transformer 7 and thereafter, is supplied to the load 11 as a fixed DC voltage.
Here, the control circuit 12 performs an ON-OFF control of the switching element 8 to set the DC voltage supplied to the load 11 to a substantially fixed value. The DC voltage supplied to the load 11 is detected by resistances 13a, 13b and the detected value is compared with a predetermined voltage set value or the like in the control circuit 12. Then, an ON-OFF duty cycle of the switching element 8 is controlled based on a PWM (Pulse Width Modulation) control or the like to eliminate the deviation between both voltages.
A charging current is supplied to the smoothing capacitor 6 from the AC power source 1 through the noise filter 2, the bridge rectifying circuit or device 5, and the choke coil 17. The charging current has its peak value suppressed in response to an inductance value of the choke coil 17 while prolonging the a current supply period. That is, since the charging current that flows into the smoothing capacitor 6 is smoothed by the choke coil 17, the power factor is improved.
FIG. 6 illustrates another switching power source device that uses a so-called PFC (Power Factor Correction) method, where a power factor is set to a value substantially equal to 1 by converting an input current into an approximately sinusoidal wave, while eliminating the higher harmonic components in the input current, as disclosed in Japanese Unexamined Patent Publication Hei-11 (1999)-196572. The embodiment of FIG. 6 is similar to that of FIG. 5, but it further includes a second switching element 14, a diode 15, a current detection resistance 16, a second control circuit 18, and an inductor 19. Here, the voltage of the smoothing capacitor 6 and the current detection value obtained by the current detection resistance 16 are input to the second control circuit 18, and an ON-OFF control of the second switching element 14 is performed based on these input signals.
The inductor 19, the second switching element 14, the diode 15, the smoothing capacitor 6, the current detection resistance 16, and the second control circuit 18 constitute a booster converter. The input current waveform is formed into a sinusoidal waveform by performing the PWM control of the switching element 14 by the control circuit 18, the higher harmonic components are removed, while improving the input power factor such that a value substantially equal to 1 is realized as the power factor.
The noise filter 2 shown in FIGS. 5 and 6 is constituted of the reactor 3 and the capacitor 4. Although not shown in the drawing, the noise filter also connects a capacitor to a power source side of the reactor 3, as is well known. This constitution is referred to as “a normal mode noise filter” and has a function of removing the normal mode noise current flowing in positive-side and negative-side output lines of the bridge rectifying circuit 5.
Further, although not shown in the drawing, an in-phase reactor provided with two windings having the same polarity on the same core can be used in place of the reactor 3, with the two windings are respectively grounded through capacitors. Such a structure is referred to as an in-plane noise filter (a common mode noise filter) and the noise filter has a function of removing the common mode noise current flowing between positive-side/negative-side output lines of the bridge rectifying circuit 5 and the ground in response to turning ON and OFF of the switching element 8.
To the choke coil 17 of the switching power source device shown in FIG. 5, a pulse current obtained by the full-wave rectification of an AC current from the AC power source 1 is applied. The frequency of the pulse current is twice as large as a commercial frequency and hence, it is necessary for the choke coil 17 to induce a large inductance value of several mH or greater. However, the choke coil having the large inductance value has a larger shape and a larger weight, which hamper the miniaturization and the reduction of weight. Further, it is necessary to increase the number of turns of the choke coil to obtain a larger inductance value, and hence, a voltage drop due to the resistance of the winding is increased, lowering the DC intermediate voltage between both ends of the smoothing capacitor 6. Accordingly, an effective current flowing in the switching element 8 is increased, and hence, several drawbacks arise, including increase in a switching loss and lowered efficiency of the switching power source device as the power source device.
Although the switching power source device shown in FIG. 6 can obtain the power factor of substantially 1, it is necessary to provide two control circuits for the switching element, and hence, the circuit constitution becomes complicated, increasing the cost. On the other hand, not many applications require the total removal of higher harmonic components contained in the input current while holding the power factor as substantially 1. Particularly, with respect to the higher harmonic components, it is sufficient to reduce the higher harmonic components to a value equal to or less than a value determined by standards or the like in many applications, and hence, the switching device shown in FIG. 6 is wasteful with respect to the functions and the cost.
Accordingly, there remains a need for a switching power source device that can obviate the increase of size and the increase of cost of the device by making a choke coil and the like unnecessary. Further, there remains a need for a switching power source device that can improve the power factor by increasing a conduction angle of an input current in a wide input voltage range. Further, there remains a need for a switching power source device that can remove higher harmonic components of an input current at a level sufficient for practical use, while achieving high operating efficiency by reducing the switching loss. The present invention addresses these needs.